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United States Patent |
5,121,870
|
Evans, Jr.
,   et al.
|
June 16, 1992
|
Method for reducing time to place a tool onto a workpiece
Abstract
In bonding a number of pads which may have different heights within a
predetermined tolerance the bonding tool (28, 24, 14, 10) is moved in slow
speed search mode (70) to contact the first pad (P.sub.1) and store the
height (H.sub.1) of the first pad (P.sub.1). After a predetermined amount
of continued overtravel (OT.sub.1) the bond is performed and the tool
raised. Operations on other pads may occur at this point. The tool is then
moved to a point above the location of the second pad. The tool is moved
down to the second pad without a search mode but to a second pad target
height (H.sub.1 +OT.sub.2) that is the sum of the sensed height of the
preceding pad and an overtravel distance. The approach of the tool to the
second pad is done at high speed (84,88) and senses and stores the point
of contact, or the actual sensed height of the second pad, for use in
determining a target height for the high speed approach to the next pad,
thereby eliminating the slow speed search required for all pads but the
first on a part.
Inventors:
|
Evans, Jr.; Daniel D. (Oceanside, CA);
Cawelti; Dale W. (Carlsbad, CA);
Gabaldon; John B. (San Diego, CA)
|
Assignee:
|
Hughes Aircraft Company (Los Angeles, CA)
|
Appl. No.:
|
692884 |
Filed:
|
April 29, 1991 |
Current U.S. Class: |
228/102; 228/103; 228/179.1 |
Intern'l Class: |
H01L 021/607 |
Field of Search: |
228/4.5,102,103,179
|
References Cited
U.S. Patent Documents
4266710 | May., 1981 | Bilane et al. | 228/8.
|
4598853 | Jul., 1986 | Hill | 228/4.
|
4817848 | Apr., 1989 | Gabaldon | 228/102.
|
5011061 | Apr., 1991 | Funatsu | 228/10.
|
Primary Examiner: Ramsey; Kenneth J.
Attorney, Agent or Firm: Gudmestad; Terie, Denson-Low; Wanda K.
Claims
We claim:
1. For use with an apparatus wherein a tool successively operates on a
number of different pads at adjacent locations, and wherein the distances,
H, of each pad relative to a reference in a predetermined direction of
approach of the tool toward the pad may vary from pad to pad, the tool
being moved to the pad in said predetermined direction, a method for
decreasing time required to move the tool, said method comprising the
steps of:
laterally positioning the tool at the location of a first pad to be
operated on by the tool,
moving the tool toward said first pad in a first velocity mode to contact
the pad and sense the distance H.sub.1, of the first pad,
storing the distance H.sub.1,
causing the tool to operate upon said first pad,
moving the tool form said first pad,
laterally moving the tool to the location of a second pad,
moving the tool in a second velocity mode toward the second pad, wherein
said second velocity is calculated from the stored distance H.sub.1 of the
first pad,
sensing the distance H.sub.2 of the second pad,
storing the distance H.sub.2 of the second pad, and
causing the tool to operate on the second pad.
2. The method of claim 1 wherein the tool is mounted on a carriage, and
including the step of moving the carriage through an amount of over travel
beyond each pad.
3. The method of claim 1 wherein said second velocity mode provides an
approach time less than the approach time provided by said first velocity
mode.
4. The method of claim 1 wherein said first velocity mode includes a slow
speed search mode providing a relatively long approach time, and wherein
said second velocity mode provides a relatively short approach time to a
target height equal to the stored distance H.sub.1 plus an overtravel
distance.
5. The method of claim 1 wherein said step of moving the tool in a first
velocity mode comprises moving the tool toward the first pad at a
relatively low constant speed search velocity, and wherein said step of
moving the tool in a second velocity mode comprises moving the tool to
said second pad without a search velocity.
6. The method of claim 1 wherein said step of moving the tool in a first
velocity mode comprises searching for said first pad at a constant slow
tool velocity, and eliminating searching for said second pad.
7. The method of claim 6 wherein said step of searching for said first pad
comprises moving the tool toward the first pad at a relatively rapid rate
to attain a search position and then moving the tool at a relatively slow
rate during said step of searching, ad wherein said step of moving the
tool toward said second pad comprises moving the tool at a relatively
rapid continuously decreasing rate without a search step.
8. The method of claim 1 wherein said second velocity mode provides a
minimum time to move the tool to said distance H.sub.1.
9. The method of claim 1 wherein said step of moving the tool in a second
velocity mode comprises moving the tool in a velocity mode that
continuously decreases until the tool reaches zero velocity at a target
distance H.sub.1 plus an overtravel distance.
10. The method of claim wherein said tool is mounted on a carriage, and
wherein said step of moving the tool toward said second pad at said first
pad distance H.sub.1 includes moving said carriage to a distance that is
the sum of said first pad distance H and an over travel distance.
11. A method for decreasing the time to place a bonding tool onto a pad
wherein the bonding tool is compliantly mounted on a carriage and is
arranged to perform a bonding operation on each of a plurality of adjacent
pads of which the pad height with respect to a reference varies within a
predetermined tolerance, said method comprising the steps of:
positioning the carriage and tool laterally at a first location above a
first one of said pads,
moving the carriage and tool downwardly toward the pad to a first pad
target height that is a search position above the pad,
moving the carriage and tool from said search position to said first pad at
a relatively slow velocity to contact the first pad and sense the height
H.sub.1 of the first pad,
storing said height H.sub.1,
moving the carriage and tool downwardly from the point of contact with the
first pad through an over travel distance,
performing a bond at said first pad,
raising and moving the carriage and tool laterally to said second location
above a second pad,
moving the carriage and tool downwardly toward a second pad and directly to
a second pad target height that is the sum of said first pad height
H.sub.1 and an over travel distance,
sensing and storing the height H.sub.2 of said second pad, and
causing said tool to bond said second pad.
12. The method of claim 11 wherein said step of moving the carriage and
tool toward said second pad comprises moving the carriage and tool with a
smooth velocity profile which comprises a continuously decreasing velocity
that attains a zero value at a target height equal to the sum of said
first pad height H.sub.1 and an overtravel distance.
13. The method of claim 12 including the step of moving the carriage and
tool toward a third pad to a third pad target height that is the sum of
said second pad height H.sub.2 and an overtravel distance.
14. A method of moving a bonding tool successively to each of a plurality
of pads P.sub.i, having variable heights H.sub.i with respect to a
reference, comprising the steps of:
positioning the tool over a first one of the pads P.sub.1,
moving the tool in a search mode to sense height H.sub.1 of the first pad
P.sub.1,
storing the height H.sub.1 of the first pad P.sub.1,
bonding the first pad P.sub.1,
moving the bonding tool to successive ones P.sub.i of said pads to perform
a bonding operation at each of said pads by moving the tool toward each
successive pad P.sub.i at the height H.sub.i-l of the preceding pad
P.sub.i-l at a subsequent velocity V.sub.i wherein each subsequent
velocity V.sub.i is calculated from the previously stored height
H.sub.i-l, and
sensing and storing the height H.sub.i of each pad P.sub.i.
15. The method of claim 14 wherein said step of moving the tool to each
successive pad P.sub.i comprises moving the tool directly to said height
H.sub.i-l.
16. The method of claim 14 wherein said step of moving the tool to each
successive pad P.sub.i comprises moving the tool to the height of the
preceding pad without a search mode.
17. The method of claim 14 wherein the tool is compliantly mounted on a
carriage, and including the step of moving the carriage through an
overtravel distance after sensing said height H.sub.1 and wherein said
step of moving the tool to each successive pad comprises moving the
carriage with a continuous velocity calculated form a target height
comprising the sum of said height H.sub.i-l of he preceding pad P.sub.i-l
and an overtravel distance.
18. A method of moving a tool in an approach direction to each of a
plurality of pads in sequence where the distances of said pads from a
reference in said approach direction vary from pad to pad, said method
comprising the steps of:
moving the tool toward a first one of said pads,
searching for and storing the distance H.sub.1 of said first pad from said
reference,
causing the tool to perform an operation at said first pad,
moving the tool away from said first pad,
establishing said distance H.sub.1 as a target distance for a second one of
said pads,
moving said tool toward said target distance H.sub.1 at said second pad
with a linearly decreasing velocity calculated from said distance H.sub.1,
sensing and storing the distance H.sub.2 of said second pad from said
reference, and
causing the tool to perform an operation at said second pad.
19. The method of claim 18 wherein said tool is movably mounted on a
carriage and wherein said step of searching comprises moving said carriage
and tool in a search mode unit said tool contacts said first pad, and
moving said carriage through a selected first pad overtravel distance
OT.sub.1 after said tool contacts said first pad.
20. The method of claim 18 wherein said tool is movably mounted on a
carriage and wherein said step of moving said tool to said target distance
H.sub.1 at said second pad comprises moving said carriage to a point
beyond said target distance H.sub.1 by a pad overtravel distance OT.sub.2.
21. The method of claim 18 wherein said tool is compliantly mounted on a
carriage and wherein said first mentioned step of storing comprises
generating a touchdown signal upon contact of said tool and said pad and
storing said distance H.sub.1 as a function of the position of said
carriage when said contact occurs.
22. The method of claim 18 wherein said tool is a bonding tool compliantly
mounted on a vertically moving carriage and adapted o perform a bonding
operation on each of said pads, one at a time, said step of searching for
and storing comprising moving the carriage downwardly toward said first
pad in a search mode until said tool contacts said first pad, storing a
signal representing said distance H.sub.1 upon contact of said tool with
said first pad, continuing to move said carriage through a first pad
overtravel distance, said step of moving said tool to said target distance
H.sub.1 at said second pad comprising moving the carriage in a continuous
linearly decreasing velocity high speed mode to a point beyond said target
distance H.sub.1 by a second pad overtravel distance, thereby causing said
tool to contact said second pad, said step of sensing the storing the
distance H.sub.2 comprising storing a signal representing sad distance
H.sub.2 upon contact of said tool with said second pad.
23. The method of claim 18 including the step of moving said tool upwardly,
away from said first pad after said tool is caused to perform an operation
at said first pad, and including the step of starting to move said tool
laterally toward said second pad as soon as said tool is moved away from
said first pad by a tolerance distance.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to controlled approach of a tool to a
workpiece, and in a specific arrangement concerns a method of reducing the
time required to sequentially place a tool onto a plurality of work sites
in sequence, when the work site dimensions relative to a reference
coordinate system vary by some acceptable tolerance from one work site to
another.
2. Description of Related Art
In many types of repetitive sequences a tool is moved to each of a series
of work sites on a part to operate on the individual work sites. The
operation performed on the work site is usually a fixed short time
duration, while the time required to properly and precisely position the
tool onto the work site may be a significantly longer time. Thus, the
total time of each operation sequence is greatly lengthened because of the
time necessary to properly position the tool. This problem is worse where
the tool must be precisely positioned with respect to the work site so
that a slow speed search mode must be employed in approach of the tool to
the work site.
The problem of time of approach of a tool to a work site is of particular
importance in various types of automatically controlled bonding
operations, including tape automated bonding, wedge bonding, and ball
bonding. In such bonding operations it is common to perform operations on
a number of closely adjacent work sites or pads, in sequence, by automatic
control of a bonding tool. Generally a bonding tool under computer control
is mounted on a carriage that moves horizontally in X and Y relative to a
work table upon which the part containing the pads to be bonded is
mounted. The bonding tool is first positioned directly over the pad to be
bonded and then moved downwardly at a rapid rate to a search height, which
is a tolerance height above the pad position. This is to ensure that the
tool will not contact the pad while the tool is moving at a high velocity.
Once at the tolerance height the tool is moved downwardly toward the pad
at a very slow, constant search velocity, until contact (touchdown)
between the bonding tool and the pad to be bonded is sensed. Motion of the
carriage that compliantly mounts the bonding tool is continued, beyond
touchdown, through a selected overtravel distance to ensure good contact
and application of a predetermined force between the tool and the pad.
Because the height of each pad varies from pad to pad, the tool is moved
through its tolerance height zone using a slow speed search mode to
approach each pad. The search mode is slow and time consuming and thus
greatly adds to the time required for performing a plurality of bonding
operations.
Accordingly, it is an object of the present invention to provide a method
of moving a tool to a workpiece that avoids or minimizes above-mentioned
problems.
SUMMARY OF THE INVENTION
In carrying out principles of the present invention in accordance with a
preferred embodiment thereof a tool is moved toward a first one of a
plurality of work sites or pads in a search mode, during which the first
pad height is sensed and saved. The tool then performs its operation on
the first pad and is moved to a position above the next pad. The sensed
height of the first pad is then used as a target distance to move the tool
at a relatively high speed, without a search mode, toward the second pad,
or toward an overtravel distance beyond the sensed first pad height, at
which target height the operation on the second pad is performed. During
motion of the tool to the second pad, the height of the second pad is
sensed and stored for use in selecting a target height for motion of the
tool toward the third pad. The sensed height of each pad is used to
determine the target height for high speed (without a search mode)
approach to the next pad. Thus, except for the first pad, no slow speed
search is needed or employed, even though pad heights are unknown and may
vary from pad to pad within a selected tolerance.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a schematic diagram of components of a bonding apparatus useful
in explaining the method of the present invention;
FIG. 2 is a diagrammatic illustration showing, to a greatly enlarged scale,
geometry of a number of work sites (pads) and velocity profiles of the
approach of a bonding tool carriage to several pads; and
FIG. 3 is a flow chart illustrating steps in the method of control of tool
motion.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is applicable to many different types of tools (other
than bonders) and to many different types of bonders. Typical automatic
bonders with which the invention is useful include those illustrated in
U.S. Pat. No. 4,598,853 of William H. Hill for Open Center Flexural Pivot
Wire Bonding Head, U.S. Pat. No. 4,718,591 of William H. Hill for Wire
Bonder With Open Center Of Motion, U.S. Pat. No. 4,817,848 of John B.
Gabaldon for Compliant Motion Servo, all of which are assigned to the
assignee of the present application. Features of such an automatic wire
bonder are also shown in a co-pending application, Ser. No. 559,737, filed
Jul. 30, 1990, for Electrical Interconnect Integrity Measuring Method of
Dale W. Cawelti, Daniel D. Evans, Jr. and John B. Gabaldon, also assigned
to the assignee of the present invention.
In general, these bonders embody a bond head carriage 10 (FIG. 1) that is
movable vertically (along a Z axis) over a workpiece mounted on a part
support 12, which may be carried by a worktable (not shown) that is
moveable horizontally under the bonding head along mutually orthogonal X
and Y axes. The carriage 10 includes a fixed sensor arm 14 that mounts a
fixed touchdown contact 16 for cooperation with a movable touchdown
contact 18 carried by a vertical post 20 that is integral with a
transducer frame 22. Transducer frame 22 carries an ultrasonic transducer
24, having mounted at its end a capillary like bonding tool 28 adapted to
apply a wire ball 30 to target workpieces carried on work support 12. The
ultrasonic transducer frame 22, transducer 24 and tool 28, together with
the post 20, are compliantly mounted, as a unit, to the carriage on the
fixed carriage arm 14 for a small amount of pivotal motion about a
horizontal pivot axis 34. A force actuator 36, fixedly carried by the
carriage, has a movable piston 38 which contacts fixed post 20, tending
the urge the latter in a counterclockwise direction about pivot 34. Thus
the bonding tool is compliantly mounted to the carriage for limited
vertical motion relative to the carriage to allow a selected amount of
overtravel, as will be described below.
The bonding tool and carriage are moved vertically up and down under
control of a system computer 40, which controls motion of the carriage
according to a program that will be described below and of which a flow
chart is illustrated in FIG. 3. Movement of the computer controlled bond
head carriage is monitored by a position encoder 42 which supplies a
carriage position feedback signal to the computer 40.
Electrically, the moving touchdown contact 18 is grounded, while the fixed
touchdown contact 16 connects to a logic circuit 44 including a resistor
46 and the input of an amplifier/logic circuit 48 which provides an output
to the computer 40.
To provide a generalized description of operation of the schematically
illustrated bonder of FIG. 1, the bond head carriage is moved relative to
the workpiece horizontally in X and Y to position the tool 28 directly
over the work site or pad to be bonded. This may be accomplished either by
motion of the worktable upon which the workpiece is mounted or by
horizontal motion of the bonder. Then the bonding tool is moved downwardly
to a bonding position on the workpiece. To this end the carriage is moved
downwardly while the compliantly mounted transducer 24, bonding tool 28
and post 20 are resiliently urged in a counterclockwise direction (as
viewed in FIG. 1) by force actuator 36, 38 to ensure contact between fixed
touchdown contact 16 and movable contact 18. The carriage is moved
downwardly until the tip of the bonding tool contacts the workpiece, at
which time the tool and transducer start to pivot about axis 34 in a
clockwise direction relative to the carriage fixed arm 14, to thereby
break the contact between the contact elements 16 and 18. This sends a
touchdown signal to the computer, signifying that the tip of the bonding
tool 18 has contacted the workpiece. The carriage continues through a
small preselected amount of overtravel distance, with the tip of the
bonding tool remaining fixedly in contact with the workpiece, thereby
causing the bonding tool transducer and fixed arm 20 to pivot further, by
a small amount, in a clockwise direction against the force exerted by
actuator 36 and its piston 38. A further description of this sequence of
operations is found in the above-identified U.S. Patents and U.S. patent
application, the disclosures of which are incorporated by this reference
as though fully set forth herein.
A predetermined amount of overtravel beyond the touchdown is desired to
assure that a predetermined amount of force will be exerted by the tool
upon the workpiece as the bonding operation is performed. Thus a selected
amount of carriage overtravel (permitted because of compliant mounting of
the tool to the carriage) after sensed touchdown is programmed into the
motion. When automatically performing a large number of computer
controlled bonds it is desirable to move the bonding tool down to the
workpiece as rapidly as possible. However, this cannot be done when the
height of the work sites or pad is unknown, that is, when pad height
varies within a predetermined tolerance. Therefore it is common practice,
before the present invention, to move the bonding tool downwardly to a
target position, e.g. a target search height, that is a small distance
above a nominal height of the pad to be bonded. This motion to the target
height is carried out rapidly, as can be seen in FIG. 2, with respect to
the pad indicated at P1.
FIG. 2 illustrates two dimensional geometry of bonding tool motion with
respect to a plurality of pads generally indicated at P.sub.1, P.sub.2,
P.sub.3 and P.sub.n. In this figure the several pads which are on a common
substrate or die are shown to have a varying height or distance from a
reference, generally indicated at R. Nominally and ideally the pads would
be all at the same precise predetermined height or distance below the
reference R. However, in actuality, the pad heights vary within a selected
tolerance T so that the difference in height from pad to pad may be as
much as the tolerance distance T, although, as can be seen in the figure,
the total variation in height from the first pad to a subsequent pad may
be greater than the tolerance T. This figure shows, only as an example,
linear variation of height from pad to pad. It will be understood that
principles of the present invention are equally applicable to other types
of pad to pad variations, provided only that such pad to pad variation is
within a known tolerance.
After making any one bond, the carriage and bonding tool are lifted from
the pad by a distance equal to a predetermined lift distance L, indicated
in FIG. 2, in order to be sure that the subsequent lateral motion of the
bonding head relative to the pad to position the bonding tool over the
next pad to be bonded will not interfere with any other pad that may be
higher (within the allowed tolerance) than the pad just bonded. Thus for
each pad the bonding carriage is moved to the lift height L and moves to
the pad with a programmed velocity profile.
This velocity profile, for the first pad, is illustrated in FIG. 2 as
having three sections. In the illustrated profiles distance along vertical
line 62 represents distance of the carriage from the pad, while distance
to the left of line 62 represents magnitude of carriage velocity in its
descent toward the pad. A first section of the first pad velocity profile,
indicated at 60, represents an increase of velocity to a maximum velocity
64 of the velocity profile, at which time the tool velocity begins to
decrease, as indicated at velocity profile leg 66, until the carriage
attains its initial target height, which is a search height, indicated at
68. The target search height 68 may be a small distance, typically 0.008
inches and is usually at the maximum tolerance height of the target pad
surface. The carriage has been moved rapidly to this target height, but
slows its speed before the tool might contact the pad. When target height
68 is reached by the carriage, the third leg 70 of the velocity profile
begins. This is a relatively slow, constant search velocity, continuing
the same downward motion of the carriage and tool, but at a much lesser
rate, until touchdown occurs. The constant search velocity of the carriage
carries the bonding tool 28 slowly downwardly from its search height until
its lower tip contacts the workpiece. Just prior to the moment of contact,
the two contacts 16, 18 are in electrical contact with one another, being
held together by the operation of the force actuator 36, 38. The point of
touchdown then is signalled by the breaking of the circuit between the two
contacts, and the programmed amount of carriage overtravel indicated in
FIG. 2 as OT.sub.1 begins. During the carriage overtravel, of course, the
bonding tool 28 remains in position, in contact with the pad, while the
carriage continues downwardly, to further pivot the ultrasonic transducer
and bonding tool in a clockwise direction while applying the programmed
force to the bonding tool through actuator 36. When the overtravel
distance has been reached, vertical motion stops (all horizontal motion
has stopped before the tool reaches search height), and the ultrasonic
transducer is energized to accomplish the bonding operation. Upon
completion of the bonding, the carriage is moved vertically through the
lift distance L to ensure clearing of the adjacent pads as the tool moves
laterally to the next adjacent pad.
The above-described operation of the motion of the bonding tool is used
only for first pad P.sub.1 in the system to be disclosed herein. However,
this first pad motion is an operation commonly carried out at present by
prior art systems for all pads to be bonded. Thus, according to prior art,
the approach of the bonding tool to each pad individually will include the
time consuming, slow constant speed search leg of its velocity profile as
indicated with respect to the pad P.sub.1. Typically, one part has many
bonding (work) sites. In our embodiments, the first site at each part is
measured. Each other site is then moved to without searching for its
height.
In accordance with the present invention, however, the time consuming, slow
speed search leg of the velocity profile is eliminated for approach of the
bonding tool to all of the pads being bonded, excepting only the first
pad. Thus, in general, approach to the first pad is as described in
connection with pad P.sub.1, utilizing the three section velocity profile
60, 66 and 70, which includes the constant slow speed search leg 70.
However, for approach of the bonding tool to a second pad, P.sub.2, a
target height is established at a height (e.g. distance from reference R),
such as indicated at point 80, that is equal to the sensed target height
H.sub.1 of the previous (first) pad P.sub.1, plus a selected overtravel
distance OT.sub.2 for the second pad. Generally, but not always, the
selected overtravel for all of the pads will be the same. Thus, to
approach the second pad, P.sub.2, the carriage is programmed to move
through a two component velocity profile, including a first velocity
profile leg 84. During this leg 84 the downward velocity of the carriage
ramps up to a maximum speed at a point 86 and then follows a second,
decreasing velocity leg 88 which attains zero velocity, that is, stops the
carriage, at point 80. Note that the maximum speed 86 attained during
approach to the second and subsequent pads may be, and preferably is,
greater than the maximum speed attained during approach to the first pad,
because no search mode is needed, and the carriage has a longer distance
to travel to its target height.
Although a linearly decreasing velocity profile leg 88 is illustrated, it
will be readily understood that the velocity profile 84, 88 may have any
selected variation provided that it requires an acceptably minimized time
for approach of the tool to the pad from the lift point to the target
height H.sub.1 +OT.sub.2, at which point the carriage stops.
Thus, in moving to the bonding position at the second pad, the bonding tool
moves with its most rapid velocity profile, without any slow search mode,
to and through the touchdown, to the target height H.sub.1 +OT.sub.2. At
touchdown the height H.sub.1 of the second pad P.sub.1 is sensed and
stored for use in the approach to the next bonding pad. In other words,
for approach to the second pad the carriage is programmed to move directly
to the target height H.sub.1 +OT.sub.2. Use of the selected overtravel
distance ensures that the tool is pressed against the workpiece with the
predetermined force applied by the force actuator 36.
After the bonding operation at pad P.sub.2 has been accomplished, the
carriage is raised through the lift distance, and, as soon as the lift
tolerance distance has been attained, lateral motion is commanded to move
the bonding tool precisely over the third pad P.sub.3. Lift tolerance may
be just when the touch contacts 16, 18 close and tool 28 lifts off pad.
Again there is established, with respect to pad P.sub.3, a target height
that is equal to the sum of the height H.sub.2 of the immediately previous
pad P.sub.2 plus the selected overtravel distance OT.sub.3 for pad
P.sub.3. The carriage is then moved downwardly with a maximized velocity
profile, to minimize the approach time of the tool to this new target
height H.sub.2 +OT.sub.3. Again the height H.sub.3 of this pad P.sub.3 is
sensed and stored. This sensing and storing of the height of a preceding
pad and then using that stored height to establish a target height for the
rapid approach to the succeeding pad is continued until all bonds have
been accomplished. The slow, time consuming search mode is carried out
only for the first pad, so that the overall time of bonding a plurality of
such pads is improved by approximately ten to sixty percent.
The above described steps are carried out by the computer 40 which is
programmed to accomplish the described motions. A flow chart of a
preferred embodiment of such a program is illustrated in FIG. 3, wherein
block 100 indicates a start and block 102 represents the initialization of
the "i" or bond location counter to "one". The tool is then moved
laterally in X and Y relative to the workpiece to position the tool
directly above the bond location "i" as indicated in block 104. The count
of the counter is then interrogated, block 106, to determine whether or
not this location is the first location or a subsequent location. If this
is the first location and the count of the location counter is equal to
one, the tool is then moved down to a predetermined search height, as
indicated in block 108. Upon attainment of the search height the slow
constant speed search is started, block 110, and upon contact of the tool
with the work site or pad, touchdown is sensed, block 112. Upon sensing
touchdown the actual height of the carriage at touchdown is stored or
logged for use at the next bond site, block 114. Now the carriage begins
to move downwardly through its selected overtravel distance, block 116.
The position of the carriage is then monitored to determine whether or not
the carriage has moved through the predetermined amount of overtravel, as
indicated in block 118. The carriage continues to move downwardly until
this interrogation signals that selected overtravel amount has been
reached, at which time carriage motion stops and the bonding operation is
initiated, as indicated at block 120, to bond the pad P.sub.1 which is the
first pad. Upon completion of the bond the carriage begins to move
upwardly toward the lift height, as indicated in block 122, while the
height of the carriage is interrogated, as indicated in block 124, to
determine whether or not the carriage has risen above the tolerance
height. The carriage continues to rise until this interrogation indicates
that the carriage has exceeded the tolerance height, at which time the
location counter is augmented, as indicated in block 128, and lateral
motion toward the next pad location may begin. Thus the carriage may
continue its vertically upward motion with no horizontal motion permitted
until it reaches the tolerance level. Only after rising above the
tolerance level, the carriage may begin to move laterally toward the next
pad location while continuing to move vertically to lift height.
A first loop has been completed and the program returns to block 104 to
position the tool above the next bond location. Interrogation of the
location counter indicates that this second location is not the first, and
then the carriage is commanded to begin its downward (Z axis) motion as
indicated in block 132. Note that after sensing and storing the height H
of the first pad, a target height for the second pad is established at a
height equal to H.sub.1 +OT.sub.2. Thus in block 132 the carriage motion
is begun to move the carriage to this precalculated target height, H.sub.1
+OT.sub.2. As the carriage moves at high speed downwardly toward the
second pad and the tool first contacts the pad, touchdown is sensed and
signalled as indicated at block 134. Upon signalling of touchdown the
actual pad height H.sub.2 of the second pad is stored as indicated in
block 136. The carriage continues its downward motion toward the target
height H.sub.1 +OT.sub.2 and when the selected target position height has
been attained, as indicated in block 118, the tool is in proper position
and bonding at the second pad may begin.
It will be seen that the described method significantly reduces the time
required to sequentially place a tool onto a plurality of different pads
where the pad dimensions relative to a reference vary within an acceptable
tolerance from one pad to the next. According to the method described
herein the tool need search for the pad height only for the first work
position, whereas at all subsequent work positions the tool is commanded
to move directly to the pad height measured at the previous work position
plus a specified overtravel. It may be noted that the overtravel amount
must be greater than the negative side quantity of the allowable
dimensional variation from pad position to pad position in order to ensure
that the tool will contact the pad.
Because the slow constant speed search velocity mode has been eliminated
and the tool and carriage may be commanded to move directly to the
selected target height, the carriage may be commanded to attain a higher
maximum velocity during its approach to all pads subsequent to the first
pad. Therefore the time advantage attained by the described method
includes both the elimination of the time consuming slow speed search mode
and the ability to employ a higher maximum velocity of the carriage during
approach to all pads after the first.
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